Add support for authentication/search timeout

##### Issue Type

- Bug Report
##### Summary

[LdapUserSearch is hard coding the LDAP search timelimit parameter](https://github.com/gocd/gocd/blob/master/server/src/com/thoughtworks/go/server/security/LdapUserSearch.java#L90) which prevents the administrator from tuning this parameter as appropriate for their environment.

I believe this is normally handled by the system property com.sun.jndi.ldap.read.timeout .

##### Steps to Reproduce

1. Find an LDAP server with over 100,000 entries with 'memberOf' overlay enabled
2. Add an LDAP filter in GoCD to use memberOf
3. wait 6 seconds

##### Expected Results
- Slow but working response

##### Actual Results
- Soft sobbing

##### Possible Fix
- Remove line 90 in above referenced file

##### Any other info
- I really do like you guys. Ignore the snark.